1. Field of the Invention
The present invention relates to a condenser-evaporator which executes heat exchange with a liquid in a first fluid chamber and a fluid in a second fluid chamber to vaporize the liquid in the first fluid chamber.
2. Description of the Related Art
a. Conventional Condenser/Evaporator
Many condenser-evaporators for use in double column rectifier of a cryogenic air separation plant, as disclosed in the Published Unexamined Japanese Patent Application No. 56-56592, are so-called plate fin type heat exchangers each employing many parallel partitions vertically separating the condenser-evaporator into two types of chambers, namely oxygen chambers each as the first fluid chamber and nitrogen chambers each as the second fluid chamber, which are alternately provided adjoining one another.
In each oxygen chamber of such a plate fin type condenser-evaporator, many vertical evaporation passages are formed by vertically providing heat exchanger plates, each evaporation passage having top and bottom ends open, with the bottom opening serving as an inlet to introduce, liquid oxygen and the top opening serving as an outlet to flow out a mixture of an oxygen gas and liquid oxygen. As the overall condenser-evaporator is immersed in liquid oxygen retained in the sump bottom space of the low pressure (LP) column of a double column rectifier, each oxygen chamber is filled with liquid oxygen and the liquid oxygen in this oxygen chamber is subjected to heat exchange with nitrogen gas in the adjoining nitrogen chamber, and the part of the liquid oxygen is vaporized into oxygen gas bubbles and rises in the evaporation passage. The liquid oxygen circulates from the inside to outside of a condenser-evaporator because of developed head pressure due to the density difference between the mixture of the vaporized gas and liquid in the oxygen chamber and the liquid around the condenser-evaporator in the sump.
The nitrogens chamber is enclosed chamber in which vertical heat exchanger plates are provided as in the oxygen chamber to form many vertical condensing passages, and it is connected to the high pressure (HP) column of the double column rectifier via headers provided at the upstream and downstream ends of the connecting passages. The nitrogen gas drawn out from the upper portion of the HP column is introduced to the condensing passages through the upper header to be subjected to heat exchange with the liquid oxygen in the adjoining evaporation passage, and the condensed liquid nitrogen is led out from the condenser evaporator through the lower header.
b. Disadvantages of A conventional Condenser-Evaporator
As mentioned above conventional condenser-evaporator is immersed in the sump of liquid oxygen at the bottom space of the LP column, and is well known as a material property that the higher the liquid pressure becomes, the higher the boiling point of the same liquid rises. The head pressure of liquid oxygen in oxygen passage is higher at a lower position than at a higher position of an oxygen passage depending on the depth of liquid oxygen. Therefore, the boiling point of liquid oxygen in the passage is higher at the lower position than at the higher position of the oxygen passage. This phenomenon is generally called the "rise of the boiling point". In liquid oxygen the rise of the boiling point is about 1.degree. C. per one meter of liquid depth. The condenser-evaporator is heat exchanged by the difference between the boiling temperature of oxygen and the condensation temperature of nitrogen according to the following equation of heat transfer. EQU Q=H.times.S.times..DELTA.T
where
Q: Heat exchanged kcal/h PA1 H: Heat transfer coefficient kcal/m.sup.2 /C PA1 S: Heat transfer area m.sup.2 PA1 .DELTA.T: Temperature difference C
The condensation temperature of nitrogen in the HP column (one example: -177.5C. at 4.8 kgf/cm.sup.2 G) is higher than the boiling point of oxygen in the LP column (one example: -179.5C. at 0.6 kgf/cm.sup.2 G). Therefore the temperature difference between both passages decreases with the rise of the boiling point of liquid oxygen which corresponds to the liquid oxygen head pressure. This means that the head pressure of liquid oxygen reduces the heat amount to be exchanged with the condenser-evaporator.
In order to obtain the required heat exchange, there are two available procedures if heat transfer coefficient H remains constant. The first procedure is to increase heat transfer area S. The secondary one is to increase the temperature difference .DELTA.T.
The first procedure leads us to the disadvantage of a larger shell diameter of the sump.
Normally, the difference between the temperature on the nitrogen side of the conventional condenser-evaporator and that on the oxygen side is designed only 1 to 2.degree. C. at the top of the condenser-evaporator, so that the mentioned rise in boiling point of the liquid oxygen causes a significant problem in the performance of the condenser-evaporator. In other words, making the passage lengths of condenser-evaporator longer in order to get larger heat transfer area results in increasing the height of the condenser-evaporator, or liquid depth of the liquid oxygen and thus raising the boiling point, resulting in a decrease in the temperature difference, so that the heat transfer area should be increased not by increasing the height of condenser-evaporators but by increasing their number. This results in making the diameter of the sump larger to include all condenser-evaporators, which makes it difficult to integrate the LP column, the sump and the HP column.
The second procedure leads us to the disadvantages of higher power consumption. That is, the operational pressure of the HP lower column should be increased to make condensation temperature higher in order to increase the temperature difference.
The operational pressure of the HP column determines the condensation temperature of saturated nitrogen at the top of this column. The higher the operational pressure of the HP column, the higher the outlet pressure of the air compressor becomes.
Since the power of an air separation plant is consumed mostly in the compression of raw air or increasing the pressure of the HP column to develop the temperature difference between nitrogen chamber and oxygen chamber of condenser-evaporator, any further lowering of the pressure of the column can reduce the power consumption. The process flow sheet of this double columned rectification system for air separation is shown in U.S. Pat. No. 4,372,764, Figures and descriptions of line 46-57 in column 2, lines 17-26, and lines 30-33 in column 3.
With the conventional structure in which a condenser-evaporator is immersed in liquid oxygen, however, the reduction in pressure of the HP column should be restricted by an amount of the rise in boiling point of the liquid oxygen by a liquid head.
Further, the heat transfer area of the conventional condenser-evaporator is designed to provide full performance for heat exchange when immersed fully in liquid oxygen. A full immersion of a condenser-evaporator requires that a large amount of liquid oxygen is stored up in the sump, which takes a long time. Before a full immersion, the condensed liquid of the liquid nitrogen serving as a reflux liquid of the HP column and an ascending vapor of the LP column by evaporation of the liquid oxygen are not sufficiently generated. As a consequence, the rectifying operation does not start, requiring an extended wait time (start up time) and resulting in a loss in power consumption. Further, safety measures in case of emergency become a big issue in that a lot of liquid oxygen spills if the sump breaks.
The liquid oxygen rising in the evaporation passage at the bottom portion of the oxygen chamber should be heated by convective heat transfer with a lower heat transfer coefficient up to a boiling onset temperature, which reduces the heat transfer efficiency of that passage. As the nitrogen chamber on the condensing side has condensing passages formed vertically and nitrogen gas flows downward while being condensed, the amount of the liquid nitrogen increases with condensing along the downstream of these condensing passages and the condensed nitrogen becomes a thick liquid film to cover the heat transfer surface of the condensing passages. This film serves as a thermal resistance layer and this reduces the heat transfer performance. Therefore this present invention is related to the condenser-evaporator improved the above-mentioned disadvantages or inconvenient matter of a conventional condenser-evaporator.